1. Field of the Invention
This invention relates to a hydraulic master cylinder used in a clutch controlling device of a vehicle or the like.
2. Description of the Related Art
Generally, in the case that gear is changed by a manual transmission in a vehicle, such a clutching operation is performed that a torque transmission from an engine to a transmission is interrupted by treading on a clutch pedal and the clutch pedal is released after gear is changed during the interruption to resume the torque transmission. A system, for example, as shown in FIGS. 6A and 6B is known as the one for realizing such a clutching operation.
In FIGS. 6A and 6B, a crank shaft 10 of an engine and a clutch shaft 12 are opposed to each other, and a flywheel 14 is fixed to the crank shaft 10, whereas a clutch plate 16 having a clutch disk 18 on its outer surface is so mounted on the outer surface of the clutch shaft 12 as to be displaceable along a longitudinal direction. A diaphragm spring 22 is mounted on the clutch shaft 12, and an elastic force thereof is transmitted to the clutch disk 18 via a pressure plate 20, whereby the clutch disk 18 is pressed against the flywheel 14 to keep the clutch coupled (see FIG. 6A).
The diaphragm spring 22 is pressed by a release fork 24 having a fulcrum of rotation at one end, and this release fork 24 is rotated as the clutch pedal 28 is treaded on. The clutch pedal 28 is mounted on a rotatable end of a pedal arm 27 rotatable about a supporting shaft 26, and the leading end of a rod 32 of a master cylinder 30 is coupled at an intermediate position of the pedal arm 27. A pressure chamber of this master cylinder 30 communicates with a pressure chamber of an operating cylinder 36 via a pipe 34, and a rod 38 of the operating cylinder 36 is coupled to a rotatable end of the release fork 24.
In this system, with the clutch pedal 28 released as shown in FIG. 6A, the elastic force of the diaphragm spring 22 is transmitted to the clutch disk 18 via the pressure plate 20 and the clutch disk 18 is pressed against the flywheel 14, whereby the clutch disk 18 and the flywheel 14 rotate together. In other words, a rotary force of the crank shaft 10 is transmitted to the clutch shaft 12 via a clutch mechanism.
As the clutch pedal 28 is treaded on as shown in FIG. 6B in this state, the rod 32 of the master cylinder 30 operates in a retracting direction and a hydraulic pressure created in the master cylinder 30 is transmitted to the operating cylinder 36 via the pipe 34. In turn, the rod 38 of the operating cylinder 36 operates in an elongating direction to rotate the release fork 24, thereby pushing a middle part of the diaphragm spring 22 toward the flywheel 14. In this way, the diaphragm spring 22 is deformed to free the pressure plate 20 and the clutch disk 18 from the pressed state. Then, the clutch disk 18 is separated from the flywheel 14 to separate the clutch shaft 12 and the crank shaft 10.
The master cylinder 30 provided in this system is internally provided with a piston 42 in a cylinder main portion and an unillustrated spring for biasing the piston 42 toward the rod 32. The rod 32 is pushed by the pedal lever 27 against a biasing force of this spring (see FIG. 6B), whereby the piston 42 functions to feed a hydraulic fluid in the cylinder main body to the operating cylinder 36. Accordingly, a clearance between the inner wall of the cylinder main body and the piston 42 needs to be fully sealed by a seal ring. However, if the pressure chamber (fluid chamber at the side of the piston 42 opposite from the rod 32; left fluid chamber in FIGS. 6A and 6B) of the master cylinder 30 is left constantly sealed by this sealing, the position of the clutch pedal 28 may undesirably change little by little due to the abrasion of the clutch disk 18.
Specifically, if the positions of the pressure plate 20 and the diaphragm spring 22 are displaced to left in FIG. 6A from the state of FIG. 6A due to the abrasion of the clutch disk 18, the positions of the rod 38 and the piston of the operating cylinder 36 are also displayed to left from their initial positions, whereby the piston 42 and the rod 32 of the master cylinder 30 connected with the operating cylinder 36 via the pipe 34, and the clutch pedal 28 are also displayed to left in FIG. 6A. Conversely, if an excessive hydraulic fluid enters the pressure chamber, this results in an excessively increased pressure in this pressure chamber.
As a means for preventing changes in the position of the piston of the master cylinder 30 and that of the pedal resulting from such a change of the clutching mechanism with time and a change of the pressure in the pressure chamber, Japanese Registered Utility Model No. 2557889 (see 10th paragraph of page 2 and FIG. 3 in this Utility Model) discloses that a filler opening used to supply oil as a hydraulic fluid is provided at side portion of a cylinder main body of a master cylinder, a check valve is incorporated into a piston in the cylinder main body, and the check valve is opened to communicate a pressure chamber with the filler opening only when the master cylinder is located at an extended position as shown in FIG. 6A (when the piston is located at a rearmost position most distanced from the filler opening).
If this master cylinder is used, the hydraulic fluid is supplied to the pressure chamber of the master cylinder 30 via the check valve and the filler opening by as much as a displacement, for example, even if the rod 38 of the operating cylinder 36 is displaced to left in FIG. 6A from the state of FIG. 6A. Thus, the position of the piston 42 and that of the clutch pedal 28 coupled to the piston 42 via the rod 32 are constantly held at specified positions.
In the master cylinder disclosed in the Utility Model, the check valve having a complicated construction needs to be incorporated into the piston since the hydraulic fluid is supplied to the pressure chamber with the piston located at the rearmost position. Further, as a means for opening the check valve when the piston is moved back to the rearmost position, it is necessary to fix a pin (“pin 8” in FIG. 3 of the Utility Model) penetrating the cylinder main body in a radial direction in the cylinder main body and to form an oblong hole (“oblong hole 6” in FIG. 3 of the Utility Model) in the piston to prevent an interference with the pin. Thus, the entire cylinder has a complicated construction and a larger number of parts, which leads to increased production costs and a difficulty in ensuring the sealing reliability.